The present invention relates to a packet switching apparatus for a data switching network and, more particularly, to a frame relay type data switching apparatus for implementing frame relaying services in an Integrated Services Digital Network (ISDN).
Frame relaying in an ISDN is a communication system in which only the function of updating headers on the basis of logical channel numbers is performed within the network while the processing associated with the retransmission of data due to data transfer errors and other faults is performed on an end-to-end basis ("ISDN PACKET SERVICES EVOLUTION" Mehmet Unsoy, IEEE 1987, A4.4.1-A4.4.5).
An example of prior art switching networks is proposed by Suzuki et al., NEC Co., in a paper entitled "High Speed Packet Switching Protocol," GLOBECOM '87, 47.3.1-47.3.5, 1987. The proposed data switching apparatus has on software a table which shows the correspondence of incoming logical channel numbers and outgoing logical channel numbers. When the arrival of data over any incoming communication channel is detected, the data are temporarily stored in a memory and the software table mentioned above is accessed by using a header of the data so as to find out an outgoing logical channel number associated with the incoming communication channel. Then, the header of the data is updated on the basis of the outgoing logical channel number, followed by processing for delivering the data to the outgoing channel.
A drawback with the prior art data switching apparatus discussed above is that the a prohibitive number of times of memory access are needed for updating the header because the correspondence of incoming and outgoing logical channel numbers are provided via software. Furthermore, since such updating is implemented by a processor, high speed processing is not achievable unless not only buffers and others associated with the channels but also the access time to the memory which stores the software table and the processor instructions are sufficiently fast.